The invention is related to the field of circuits for generating clock signals.
Various types of clock oscillator circuits for generating clock signals are known. Classes of such circuits include so-called resistor-capacitor (RC) oscillators, in which the oscillation frequency is determined by an RC time constant and the circuit operates by alternately charging and discharging the capacitor. Also included are so-called crystal oscillator circuits, which exploit the piezo-electric properties of a crystal to generate a clock signal of a generally very precise frequency.
It is also known to incorporate portions of oscillator circuits such as RC or crystal oscillator circuits in a semiconductor device. Typically, the timing components such as the resistor and/or capacitor for an RC oscillator, or the crystal for a crystal oscillator, reside off-chip. This requires the use of one or more input/output pins of the chip to interconnect these timing components with the on-chip circuitry.
For various reasons including cost, it is generally desired to minimize the number of input/output pins used for the various functions of a chip. It is also desirable that semiconductor devices be as simple and flexible in their use as possible. These goals extend to the clock generating function.